1. Field of the Invention
The invention relates to an organic light emitting display device using a pixel, and more particularly a pixel that can display an image having uniform luminance, and an organic light emitting display device using the pixel.
2. Description of the Related Art
Recently, flat panel displays which make it possible to reduce the faults, the weight and the volume of cathode ray tube have been developed. Typical flat panel displays are a liquid crystal display, a field emission display, a plasma display panel, and an organic light emitting display device, etc.
The organic light emitting display device displays an image using an organic light emitting diode which produces light by recombining an electrode and a hole. The organic light emitting display device has an advantage in that it has high response speed and is driven by low power.
FIG. 1 is a circuit diagram illustrating a pixel of an organic light emitting display device.
Referring to FIG. 1, a pixel 4 of an organic light emitting display device includes: an organic light emitting diode OLED; and a pixel circuit 2 connected to a data line Dm and a scan line Sn for controlling the organic light emitting diode OLED.
The anode electrode of the organic light emitting diode OLED is connected to the pixel circuit 2 and the cathode electrode is connected to a second power supply ELVSS. The organic light emitting diode OLED produces light with predetermined luminance in response to the current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the organic light emitting diode OLED in response to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. For this configuration, the pixel circuit 2 includes: a second transistor M2 connected between a first power supply ELVDD and the organic light emitting diode OLED; a first transistor M1 connected between the second transistor M2, the data line Dm, and the scan line Sn; and a storage capacitor Cst connected between a gate electrode and a first electrode of the second transistor M2.
A gate electrode of the first transistor M1 is connected to the scan line Sn and a first electrode of the first transistor M1 is connected to the data line Dm. Furthermore, a second electrode of the first transistor M1 is connected to one terminal of the storage capacitor Cst. In this configuration, the first electrode of first transistor M1 is either a source electrode or a drain electrode, and the second electrode of the first transistor M1 is the other of a source electrode and a drain electrode. For example, when the first electrode is the source electrode, the second electrode is the drain electrode. The first transistor M1, connected to the scan line Sn and the data line Dm, is turned on and supplies a data signal, which is supplied through the data line Dm, to the storage capacitor Cst. In this operation, the storage capacitor Cst is charged with a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is connected to one terminal of the storage capacitor Cst, and the first electrode of the second transistor M2 is connected to the first power supply ELVDD and to the other terminal of the storage capacitor Cst. Furthermore, the second electrode of the second transistor M2 is connected to the anode of the organic light emitting diode OLED. The second transistor M2 controls the amount of current flowing from the first power supply ELVDD to the second power supply ELVSS through the organic light emitting diode OLED in response to the voltage value stored in the storage capacitor Cst. In the configuration, the organic light emitting diode OLED emits light corresponding to the amount of current supplied from the second transistor M2.
However, the pixel 4 of the organic light emitting display device cannot display an image with uniform luminance. To be more specific, the second transistor M2 (driving transistor) in such pixels 4 has a different threshold voltage for each pixel 4 due to process variation. As the threshold voltages of the driving transistors are different, light with different luminance is generated by the difference in the threshold voltage of the driving transistors, even if data signals corresponding to the same gradation are supplied to the pixels 4.